1. Field of the Invention
The present invention relates to a charcoal canister, used in automobile, motorcycles and the like, for adsorbing fuel vapor.
2. Description of Related Art
Conventional activated charcoal canister which is packed with activated charcoal adsorbs fuel vapor evaporated from a fuel tank under high temperature when an engine is not running. With the engine running, the fuel vapor which is adsorbed by the canister is desorbed and led into combustion chambers of the engine by air which is led into the canister by negative pressure of the engine.
The fuel vapor which is adsorbed by the canister moves inside a pore of the activated charcoal and diffuses through one pore to another between neighboring activated charcoal as time elapses. Furthermore, when fuel of the fuel tank contracts with decreasing temperature, an air flow inside the canister (back purging) is caused by decreased pressure in the fuel tank. This back purging causes a desorption from the pores and a diffusion into the air of the adsorbed fuel vapor. Therefore, the fuel vapor may be released into the air even before the canister reaches the theoretical limit of adsorbing capacity.
In order to overcome the above-mentioned problem, JP-A-5-33734 and JP-U-63-198464 disclose canisters as shown in FIG. 6 and FIG. 7 respectively. In FIG. 6, a downstream end of a main canister 4 which is equipped with a fuel vapor inlet port 41 and a purging port 42, and an upstream end of an additional canister 5 which is equipped with an air inlet port 51 are connected by an air layer 6. In FIG. 7, an air layer 8 is provided between a first activated charcoal layer 71 and a second activated charcoal layer 72. In FIG. 7, the canister has a fuel vapor inlet port 91, a purging port 92 and an air inlet port 93. These conventional canisters inhibit the diffusion of fuel vapor by dividing the activated charcoal canisters with the air layers.
However, according the conventional canister shown in FIG. 6, complexity of the structure of the canister causes a decrease in productivity and an increase in cost because the number of parts increases. Furthermore, it may be difficult to achieve a low pressure loss structure to clear the new fuel vapor regulation in U.S.A. (ORVR) because the pressure loss is high. According to the conventional canister shown in FIG. 7, the canister becomes larger because the air layer requires a large space to prevent the diffusion of the fuel vapor.